I couldn't detect anything unusual.
When a compressor is working it does tend to 'knock' as it compresses the air (it's hard work - like an engine in reverse). The air should pass directly to the first 'receiver' (usually a condensing tank) before moving on to the air reservoirs.
The gauges will be on the reservoirs - not the condensing tank. The signal for the unloader/governor valve will probably be taken from the condensing tank (via a small diameter plastic pipe) and there will be non-return valves between the condensing tank and the air reservoirs.
Usually (in Britain) the unloader/governor valve is not attached directly to the compressor but sends its signal to unload the compressor via a small diameter plastic pipe.
As I've stated, disconnecting the unloader/governor valve pipes should allow the compressor to 'charge'. It could be that the unloader piston in the compressor is stuck in the 'open' position (though this would not be the case if the vehicle was left without air in the system - only if it had been left fully-charged with
no leaks).
In normal operation it requires air from the unloader/governor valve (from a fully-charged system) to operate the unloader in the compressor. This only happens when the pressure in the unloader/governor valve pushes the valve against the spring. An internal leak within the unloader/governor valve (it has pistons and seals) might feed air directly to the compressor, but disconnecting the unloader/governor valve piping to the compressor should tell you if that is the case (and should start the compressor working again).
It could be that your friends are talking about the unloader mechanism in the compressor rather than the (usually) separate unloader (governor) valve. In which case the problem might lie within the compressor cylinder head (the piston which depresses the inlet valve to stop it compressing air). If that is the case then you are looking at repairing the compressor (new seals in the unloader piston).
You should be able to dismantle the unloader piston without removing the compressor cylinder head (you will need seals for the piston and maybe new spring(s) if broken (unlikely I'd say), but if the inlet valves are not seating then it's a (compressor) head-off job - no need to remove the compressor - that would be a
major major job as no doubt the fuel injection pump is driven from the back of the compressor?
Do the diagnostics first, as you need to know where the problem lies before you start pulling things apart!
Watch the videos in:-
http://www.truckandbusforum.com/showthread.php?t=4674